A Group 3-5 nitride semiconductor has been used for making a semiconductor device applied to various display devices. For example, as a material for a light-emitting device such as an ultraviolet or blue light-emitting diode, or a laser diode, or for a high-output or high-frequency electronic device, a compound semiconductor represented by a general formula: InxGayAlzN, wherein x+y+z=1, 0≦x≦1, 0≦y≦1 and 0≦z≦1, are known. The compound semiconductor will be abbreviated as “a Group 3-5 nitride semiconductor”.
By the way, although a method of forming the light-emitting device comprising a Group 3-5 nitride semiconductor on a substrate of sapphire or the like is well known, when a substrate made of a material different from a Group 3-5 nitride semiconductor, sapphire or the like, is used, the substrate and the Group 3-5 nitride semiconductor are different in lattice constant or thermal expansion coefficient from each other, and thus there is such a problem that a high-density dislocation occurs after the Group 3-5 nitride semiconductor has been epitaxially grown, or that warpage occurs on the substrate, and in the worst case, it causes a crack. For this reason, a double-hetero structure Group 3-5 nitride semiconductor having an n-type Group 3-5 nitride semiconductor layer, a Group 3-5 nitride semiconductor layer as an active layer, and a p-type Group 3-5 nitride semiconductor layer on a free-standing substrate of a Group 3-5 nitride semiconductor is proposed. It is disclosed that this semiconductor is grown on the free-standing substrate of a Group 3-5 nitride semiconductor having low-dislocation densities, and thus it is excellent in crystallinity, and provides a light-emitting device wherein deterioration in emission properties is controlled (for example, JP-A-2000-223743). However, at this time, no bulk substrate made of a Group 3-5 nitride semiconductor can be industrially and inexpensively provided.
Accordingly, a method of obtaining a high-crystallinity substrate, comprising the step of once growing a Group 3-5 nitride semiconductor over a sapphire substrate, followed by selective growth over the Group 3-5 nitride semiconductor by using a SiO2 mask, is proposed (for example, JP-A-2002-170778).
Further, a method of obtaining a Group 3-5 nitride semiconductor separated from a base substrate of sapphire or the like, comprising the step of growing the Group 3-5 nitride semiconductor over the base substrate, is proposed. For example, JP-A-2001-53056 discloses a method of separating a base substrate and a GaN layer from each other, comprising the steps of: once growing the GaN layer over the base substrate; forming SiO2 stripes over the GaN layer; and regrowing an additional GaN-layer thereover; and thereafter partially etching the additional GaN-layer to the SiO2 stripes to form a trench structure; further forming a GaN layer over the trench structure to planarize the surface of the GaN layer, followed by introducing an etching liquid into the trench structure to etch the SiO2 stripes.
However, in order to form a GaN layer of a high-crystallinity Group 3-5 nitride semiconductor over the sapphire substrate according to the method disclosed in JP-A-2002-170778, the following steps are required, that is, the steps of: carrying out what is called two-step growth method which comprises forming a low-temperature buffer layer over the sapphire substrate, and forming a GaN layer thereover at a high temperature of 1000° C. or more; once unloading the Group 3-5 nitride semiconductor multilayer substrate from a semiconductor-crystal growing apparatus; forming the SiO2-stripes mask over the Group 3-5 nitride semiconductor layer; reloading it into the semiconductor-crystal growing apparatus, followed by forming a Group 3-5 nitride semiconductor at a high temperature of 1000° C. or more. Thus, the method disclosed in JP-A-2002-170778 needs to carry out the crystal growth which requires a high temperature of 1000° C. or more for many hours two times.
On the other hand, in order to obtain the Group 3-5 nitride semiconductor substrate according to the method disclosed in JP-A-2001-53056, the following steps are required, that is, the steps of: processing the trench structure by using photo-lithography which needs steps such as resist exposure; and further carrying out the GaN crystal growth at a high temperature of 1000° C. or more for many hours over three times.
Thus each of JP-A-2002-170778 and JP-A-2001-53056 has the problem that it needs a long processing time at high production cost. Accordingly, a high-performance Group 3-5 nitride semiconductor light-emitting device could not be produced at a low price.